Heretofore, copy machines have identified documents which may not legally be copied, such as bank notes, by finding a specified image on them (for example, a circle with the word "secret" in its center) or by determining the size and other features of the document.
With these types of existing methods, it sometimes happens that shapes similar to the marks that prohibit copying are found on an ordinary document, or the size of the document may be nearly identical to that of one which may not be copied. When this occurs, the document which one wishes to copy will be treated as non-reproducible, and it will not be copied. The method generally used to determine whether the document has marks on it cause the marks to stand out from the background by processing the image to separate the colors. However, when the marks and the design around them are printed in similar colors, it becomes difficult to separate and highlight only the marks and none of the surrounding image. As a result, some non-reproducible documents may not be identified as such.
Another problem occurs when the marks are of the same shape but different colors or when they are of different shapes and different colors. In these cases, we must provide as many determination processing circuits and binary memories as there are colors of marks in order to binarize the image by color separation.